Viticultural Geology

How what's below the surface affects wine quality

by
William A. Fuchs

Click here to view the full-size version of this map. Source: Graphics: JBA Works Inc.; CAD: T. Zitko

This is a tale of three hillside vineyards in Napa, Calif.

I am a geologist who has lived in Napa County for eight years. I know rocks—especially the rocks of a small area of west Napa Valley, where I mapped the geology of the vineyards of Trefethen, Darms Lane and Shifflett Estate in great detail. A few years later, the managers of these vineyards told me that they saw correlations between the geology I mapped and the origins of their better wines.

Yes, this is an article about terroir, but it emphasizes the mapping of rocks. I mapped soil also, but not from the perspective of a soil scientist, rather from the perspective of geomorphology, the study of landforms.

Soils generally have a rock component in them, and it is a general truth that the rock component typically becomes dominant the closer you are to rock outcrops—and, in the case of Napa, on the hillsides. There, the rock component is not only dominant but also more closely tied to the immediate rock geology.

In the valleys, the rock component of soils may or may not be dominant, as compared to organics and clays produced by weathering, but it is much less tied to the local rock outcroppings. Valley soils are to my mind more complex and homogenized, and some of them are more mature. My point here (and one of the main themes of this article) is that by mapping the rocks on a detailed scale on the hillsides, you have in your hands maybe 60%-80% of the important information you need about the soils. Important to what? Well to wine, of course!

Janet and John Trefethen supported the cost of some analytical work to create a geologic map of a portion of the Trefethen Hillspring Vineyard. Source: Graphics: JBA Works Inc.; CAD: T. Zitko

Starting at TrefethenI started my project at the Trefethens’ Hillspring Vineyard (Trefethen Family Vineyards), roughly 1.5 miles north of the city of Napa, where the owners, Janet and John Trefethen, were good enough to give me access to their ground and support the cost of some analytical work. After Trefethen, I continued mapping southward, where I had the cooperation of Larry Bump and his daughter, Tricia Bump Davis, of Darms Lane (formerly Crichton Hall Vineyard), and then on to Jeffrey Shifflett’s ground (Shifflett Estate.) The results of all this, in somewhat simplified and reduced form, are the three separate maps in this issue (one above and two below), portions of the original full-size map.

Having mapped during my career in places such as New Guinea, Turkey, Mexico, Alaska and the Canadian Arctic, I expected Napa to be pretty tame. For the most part it was, but I met up with rattlesnakes, poison oak, ticks (one of which nailed me in the belly button, creating an infection that sent me to the emergency room), coyotes and mountain lions—OK maybe not the last one on this list, but it seemed as though they should be there. The experience was wilder than I expected.

The geology was very interesting, partly because of the surprisingly large number of rock units. Most people are not aware that much of west Napa Valley is incompletely mapped. I work for a wine cave builder and perform initial site investigations (we like some rocks for cave building and others not so much.)

The state survey mapping is more complete further west in Sonoma County, in the Mayacamas Mountains east of the cities of Sonoma and Santa Rosa. I mapped at a scale of 1:2,400 (or 1 inch=200 feet.) In the field of geologic mapping, scale is not an abstract concept; it is of premier importance. You must choose the right scale to accomplish a certain objective. To get information that can be correlated with vineyard/wine quality, I believe a scale of 1:2,400 is the right choice. It is a somewhat unusually detailed scale for mapping rocks, though not necessarily unusual for a soil scientist.

Contour interval is important as well. This gives you the vertical component on a map, and a good geologist can look at a topographic map and see the ground in three dimensions. The topographic map is the base upon which the mapped geology is laid. A good contour interval at this scale is 2 or 5 feet, and it had better be a custom-made topographic map.

Sonoma Volcanics
Napa has a variety of geologic terranes (areas with distinctive rock types and geological history), but the hillside vineyards I mapped are all in the Sonoma Volcanics, as is much of the valley.

I like to imagine what the place would have been like at the time the rock was formed, though in this spot you would likely have been dead within minutes or seconds. The volcanism at the time was as spectacular as anything we have ever seen. The Sonoma Volcanics were formed between 2 million and 7 million years ago. Human evolution began about 7 million years ago, so these rocks span most of human evolution (except the past 2 million years.) It was a busy time for humans, but nobody was here in California (although there’s some uncertainty about that point.) Most of the valley rocks span a more restricted interval of 2.7 million to 5.5 million years. It sounds like long ago, but to a geologist that was only yesterday—not that old at all.

Since about 2 million years ago, Napa has mostly been eroding away, but the area is seismically active as well. Tectonically, the valley is known as a pull-apart basin, a geologic feature that is well known to petroleum exploration. But there’s no oil here; the rocks are wrong. Complex interactions along faults, some still potentially active, expanded the area and down-dropped the rocks to form the basin that is Napa Valley. There are also horizontal motions along the faults.

Some uplift in the mountains is probably still ongoing, and there is a battle between erosion, uplift and down-dropping that is being orchestrated by the plate tectonics of California. Nature is messy, and there is much geologic complexity in th is area. There is some folding, but the mapping I did shows that block faulting with tilted (but unfolded) blocks bounded by faults is the norm on these three vineyards.

Traversing the vineyards that I mapped is a very large fault that was previously unrecognized. I named it the Trefethen fault and determined that it has hundreds of feet of vertical slip (potentially more than a thousand feet) with a sense of movement that is down to the east. The Trefethen fault has had a major influence on the distribution of rock types and appears to be a branch of the more famous West Napa fault located just east of my map and those pictured here.

One important aspect of geology is stratigraphy, the sequence of rocks from older to younger in a layer-cake succession. Knowing the stratigraphy is helpful in determining the structure—the latter dealing more with faults, folds and dynamic deformation. Some rocks have no internal stratigraphy, such as the granites in the Sierra Nevada Range. Sedimentary rocks have stratigraphy, and so do volcanics.

The most important initial step in mapping is to select mappable units. I have eight stratigraphic units mapped on these properties, where before there was only one: Sonoma Volcanics undifferentiated. I found the units I defined to be mappable, and each is represented by a different color on the map. Importantly, some of these stratigraphic units were found to correlate with good wines, so I will focus on those units.

Whole-rock analyses of samplesWhat helped to refine and define the various units mapped was obtaining whole-rock chemical analyses for many rock samples collected from the site. Whole-rock refers to the main chemical constituents in a rock such that they add up to 100% of the rock (in contrast to trace elements, which are present in very small amounts.)

Volcanic rocks can be classified on the basis of whole-rock analyses, mineralogically or based on field classification. Because whole-rock analyses are expensive, most mapping is based on field classification, but I found that whole-rock analyses were essential for the classification of these particular rocks. Microscopic analyses also were important in determining rock type, though somewhat less so than the chemical analyses.

I wrestled a little with how to define wine quality for the purposes of this project. It appeared to me that it was best to confine vineyard plot comparisons to within each of the three entities (Trefethen, Darms Lane and Shifflett), rather than comparisons between the three entities or even Napa vineyards in general.

In this way, I could rely on the individual owners/managers to make the determinations. After all, who knows the relative quality of the vineyard plot wines better than those closest to them? The subject of wine quality still would seem difficult when you consider the different variables (vine ages, cultivars, rootstocks and more within a single operation), but the managers of all three of these operations seemed to have no difficulty identifying the vineyard plots that produced their best wines.

So there you have it: no point system, no fancy quantitative methods. I just accept what they tell me about their wines (with a small amount of tasting by myself to keep it honest.) An award-winning wine, of course, has to be mentioned, and I do so with no apology.

A geologic map of Shifflett Estate Vineyards covers the area where five Bordeaux varieties grow. Source: Graphics: JBA Works Inc.; CAD: T. Zitko

Shifflett Estate VineyardsJeffrey Shifflett produces all five red Bordeaux grape varieties on 60 planted acres, but most of the production is Cabernet Sauvignon and Merlot. He definitively singles out three vineyard blocks. His best is F block: high on a hill, which is planted with Cabernet Sauvignon Selection 7 on St. George rootstock (he says this is unusual.) It produces a “killer” wine that has done well for him since 1998.

No doubt climatological conditions and the right vines matched to the right soil are important, but it is remarkable how well the block location corresponds to a dominant dacite unit lithology. The dacite consists mostly of fine-grained volcanic flows. Chemically, dacite has a fairly high silica (SiO2) content. The soil is thin on F block, and the underlying rock breaks into small chunks. Drainage is naturally good.

Nature has provided for Shifflett in multiple ways on this small hill. The winegrower often ends his tastings by hauling customers up the hill in the bed of his rickety pickup truck to see the great view. This informal, almost counter-culture Napa experience sets Shifflett apart and makes for an appealing and memorable visit. I saw him bring a lot of happy people up the hill while I was mapping in the field, and it brought home the fact that wine is about more than a drink.

Second best for Shifflett is block B1, a Cabernet Sauvignon Selection 6 (the Jackson clone) on 110R rootstock. This excellent block resides on a rock unit of andesite-dacite and soil derived from it and transported just downhill. This rock that spans the compositional fields of both andesite and dacite has less silica than the dacite of block F, but it is also a volcanic flow rock. A distinctive rock, it can be characterized as containing abundant, larger feldspar crystals (18%-27%) set in a medium to dark gray and even blackish matrix that is very fine grained and sometimes glassy.

This is one of the dominant rock types of the hillsides west of Napa Valley, extending from Shifflett to at least a couple miles further north, near the Veterans’ Home at Yountville. On the east side of the valley it is found in the Stags Leap area. A soil with abundant hard blocks of various sizes is typical. Cabernet Sauvignon at Shifflett Estate seems to do better in this rock than Cabernet Franc, Merlot or Syrah.

Third best—and still outstanding—is block A (with sub-blocks A1, A3A and A3B.) Shifflett produces Cabernet Sauvignon here: for example, Selection 337 on rootstock 1103P in sub-block A1. The dominant unit is alluvial fan. Alluvial fans consist of soil and transported rock formed from ultra-flood-stage stream flow and deposited at (and downslope from) the gradient break at the mouth of a canyon.

Darms Lane VineyardDarms Lane was formerly known as Crichton Hall Vineyard. The previous Chardonnay vineyard was replanted beginning in 2002, primarily with Cabernet Sauvignon as well as four of the five Bordeaux varieties (Malbec was not included.) There are 14 acres planted. The first vintage of Darms Lane Cabernet Sauvignon was produced in 2005 and released in 2008. Cabernet Sauvignon is king here, of course. It is a young vineyard, but already it is apparent to the owner and winemaker where the better wine-producing plots lie, and these are all planted with Cabernet Sauvignon.

After partial ownership in the property dating back to 1990, Larry and Linda Bump transitioned to 100% ownership in 2003, and their daughter Tricia Bump Davis now runs the show day-to-day, with Larry’s role being that of strategist. The winemaker is Brian Mox, a chemist-turned-winemaker. Mox has concentrated his efforts for Darms Lane on producing wines that naturally reflect the vineyard soils.

According to Mox and Bump Davis, the best quality wines come from block B (planted with clone 337 on rootstock 110R), which coincides very nicely with what I mapped as a basalt-basaltic andesite unit just in front and east of the tasting room. This rock unit has higher calcium, magnesium and iron than the others, with the calcium and magnesium (the latter in moderation) typically being considered an asset for grapegrowing. Iron—with its complex biogeochemical behavior (especially in the presence of calcium), and not being universally recognized as a highly important constituent (other than as a micronutrient)—seems anecdotally to have higher importance as a nutrient for high wine quality within the Sonoma Volcanics. Basalt-basaltic andesite is a fine-grained, dark (nearly black), formerly molten rock that occurs in dikes (i.e., tabular slabs of rock that are younger than, and cut across, the other rock units.) Unlike the other volcanic units, this rock probably never made it to the surface when it was in its molten state, but it may have served as a feeder system to other volcanic rocks that have long since eroded away. The sliver of basalt-basaltic andesite dike that crosses Darms Lane, and which came up the aforementioned Trefethen fault, is small but had a great impact in producing Darms Lane Bon Passe Cabernet, the pride of the vineyard.

Second best is block E (planted with clone 4 on rootstock 110R), which produces grapes for another Cabernet Sauvignon called Linda’s Hillside. Block E is traversed by a contact between two rock types, namely basaltic andesite-andesite and basalt-basaltic andesite, so the respective contributions to wine quality are not so clear-cut—basalt-basaltic andesite being established as good as per Block B, but the basaltic andesite-andesite not otherwise clearly a good rock unit. Since the wine is a blend between the two, possibly the basalt-basaltic andesite is contributing more to the quality. This is speculation, however, because it is the only south-facing slope on the property, and maybe this was an equally important factor. Geology can be messy but, in this case, the fact that the vineyard is sourced from two lithologies is an artificial problem that complicates the analysis.

Third best is Cabernet Sauvignon from blocks C9-C12 (planted with clone 7 on rootstock 101-14), and within these blocks the better quality is on the west side, more on the hillside. The favorable unit here is the dacite, same as the favorable dacite unit at Shifflett, and colluvial soil (slope wash) derived from dacite.

Trefethen Family VineyardsTrefethen Family Vineyards is owned by Janet and John Trefethen, well-known personalities in Napa and a major force behind establishing the Oak Knoll District of the Napa Valley AVA. Although the district is most noted for Merlot in the reds category, two out of the three vineyards I mapped have an emphasis on Cabernet Sauvignon (with Shifflett having about equal emphasis on Merlot and Cabernet Sauvignon.) The Trefethen Hillspring Vineyard contains 25 planted acres plus 15 fallow acres within a much larger block of ground. However, the hillside has relatively small production compared to the nearby Trefethen main ranch on the valley floor.

Jon Ruel is the director of viticulture & winemaking, COO of Trefethen and my source for information about the performance of various vineyard blocks. Trefethen is a large operation with 100 full-time employees. Peter Luthi was the winemaker prior to 2009, and Zeke Neeley has been Trefethen’s winemaker in the years since.

All the best vineyard plots at Trefethen’s Hillspring Vineyard are planted in Cabernet Sauvignon, just as at Darms Lane and Shifflett. Ruel singled out block 807CS as Trefethen’s best vineyard plot. (See map above.) This lies on the same basalt-basaltic andesite unit that represents the best and second best vineyard blocks at Darms Lane. As the Shifflett ground does not have outcrops of this unit (and hence no ranking here), this appears to be a star geologic unit.

I attribute this to the chemistry and mineralogy of the rock type, which is geologically considered somewhat mafic but not ultramafic—the latter being universally considered bad from an agricultural viewpoint. Higher calcium, magnesium and iron are likely the “agriculturally critical” elemental characteristics of basalt-basaltic andesite that distinguish this rock as a viticultural superstar, but it seems to weather into a nice soil as well.

Second best for Ruel are the small contiguous vineyard plots designated 803PV (planted with Petit Verdot), 803CF (Cabernet Franc) and 803CS (Cabernet Sauvignon), which lie within the rhyolite flow unit and (to a lesser extent) the dacite unit. The dacite relationship is easy to accept, as it is the best vineyard plot unit at Shifflett and third best at Darms Lane; however, the rhyolite flow for second best was more difficult for me to fathom. There’s nothing like facts to mess up the story.

Rhyol ite lies on the other end of the chemical spectrum from basalt, the previously discussed star performer. The basalt-basaltic andesite contains an average calcium content of 9.87% CaO and iron content of 9.49% Fe2O3, while rhyolite flow contains an average calcium content of 0.53% CaO, an iron content of 2.60% Fe2O3, and it is also very low in magnesium. This rhyolite flow unit, with its lower calcium, iron and magnesium (but higher potassium) content, does not occur at Darms Lane or Shifflett, so the evidence is all at Trefethen.

It stands as proof that the story is not all about rock chemistry and mineralogy. There are other important characteristics of rock that include fabric (or lack of fabric), fracturing, grain integrity, hydrothermal and weathering alteration—and even geophysical characteristics such as conductivity and magnetism, all of which can interact with the biosphere in ways that we are only (hopefully) beginning to understand. With all these variables, it is little wonder we lack a full understanding of why a grapevine of certain genetic makeup does better when growing upon a certain rock unit.

Trefethen’s third best vineyard plot, a portion of 801CS, lies largely on alluvial fan, just as at Shifflett. Alluvial fans along the west side of Napa Valley are clearly good places to grow Cabernet Sauvignon—something Napa growers should note. The adjacent, southeastern-most part of the alluvial fan, a plot designated 897CS, is where the majority of grapes for the award-winning Trefethen 2005 Reserve Cabernet Sauvignon were sourced. Just to mess up the picture a little, however, Ruel says that since that spectacular vintage, the plot has not performed as well. Many diverse factors including geology must have converged in 2005 to create an excellent wine, and what happened once may happen again.

A non-geological factor that is apparent at all three vineyards (Trefethen, Darms Lane and Shifflett) is that south-facing slopes do better for Cabernet Sauvignon than those facing in other directions, and here I must bow to microclimate as being nearly equal to geology as a factor. The proof of this is at Trefethen where, within the basalt-basaltic andesite, the largely southeast-facing slope behind the Trefethens’ house is a prime performer while in the same rock unit further south, on a northwest-facing slope, the vineyard does not perform as well.

ConclusionRock outcrop/sub-crop geology, not just soil, is of major importance in the determination of wine quality on the hillside vineyards of Napa. If mapped in detail, the geology can be used as a predictive tool in vineyard management.

California professional geologist Dr. William Fuchs (PG 7695) obtained a master’s degree in geochemistry from Penn State University and a Ph.D. in structural geology from the University of Utah. A consulting geologist with an emphasis on mapping and prospect evaluation for the mining industry, Fuchs has mapped more than 200 square miles in his career and worked in the Napa, Calif., area for wine cave builder Magorian Mine Services.